Groeneveld et al.: Escapement of Jasus lalandu from traps 



61 



evenly with square openings across the metal trap- 

 frame and because we made sure that all lobsters 

 that could escape, did, resulting in a tight SR of 2.6 

 mm. This L 50 is remarkably close to the present MLS 

 of 75 mm CL for the commercial fishery, especially 

 considering that 100-mm mesh was first used when 

 the MLS was 89 mm CL, and that the commercial 

 mesh size remained at 100 mm despite the 14 mm CL 

 reduction in MLS during the early 1990s (Schoeman 

 et al., 2002b). The L 50 obtained from the field trials 

 with sealed trap openings (77.4 mm) was also close to 

 the present MLS. 



In a recent indirect study (i.e., where the size com- 

 position of a population was unknown) Schoeman et al. 

 (2002a) found L- to be 79.2 mm (SJR=11.1 mm) under 

 commercial operational conditions. The increase in L 50 

 (above the 75.1 mm and 77.4 mm found in the direct 

 aquarium and field studies, respectively) is the result of 

 the trap entrances of commercial traps remaining open, 

 so that rock lobsters that are too large to fit through 

 the mesh can still escape through the entrance. In the 

 present direct study, this factor increased the L 50 from 

 77.5 mm (sealed entrance) to 82.2 mm (open entrance) 

 for 100-mm mesh. Thus, one conclusion of the indirect 

 study, namely that the South African fishery for J. 

 lalandii is unusual in that standard commercial traps 

 are covered with mesh having an aperture considerably 

 wider (L 50 =79.2 mm CL) than that required to retain 

 Cape rock lobsters of the current MLS (Schoeman et 

 al., 2002a), must now be seen in a different light. The 

 selectivity of the 100-mm stretched mesh itself now 

 appears not to be wider than that which is currently 

 required (based on the direct results). Rather, the indi- 

 rectly determined L 50 appears to have been inflated by 

 the numbers of larger lobsters that were able to escape 

 through the trap entrance. 



Direct studies of the escapement of crustaceans from 

 pots (Krouse and Thomas, 1975; Krouse, 1978; Everson 

 et al., 1992) have often been criticized because these 

 studies themselves may affect the behavior of the ani- 

 mals and do not include the dynamics of the processes 

 of entry and escapement (Xu and Millar, 1993; Treble 

 et al., 1998). We recognize these weaknesses, but felt 

 that direct studies remain useful because they can be 

 used to set a theoretical benchmark against which the 

 results of indirect studies can be tested, especially if 

 the trap selectivity of the latter depends on area and 

 season. Various insights were gained from the pres- 

 ent study, particularly because it closely followed an 

 indirect study of trap selectivity for J. lalandii (Schoe- 

 man et al., 2002a). In conclusion, this study of escape- 

 ment of J. lalandii through square meshes showed 1) 

 that 100-mm mesh size is, theoretically, near optimal 

 for the fishery; 2) that many Cape rock lobsters that 

 are able to escape through the mesh do not do so; 3) 

 that the rock lobsters that are shown theoretically to 

 be unable to escape through the mesh of commercial 

 traps, often can do so; and 4) that specimens too large 

 to escape through the mesh can escape through the 

 trap entrance. 



Acknowledgments 



This study would not have been possible without the 

 funding and infrastructure provided by Marine and 

 Coastal Management (Department of Environmental 

 Affairs and Tourism, South Africa). In particular, we 

 would like to thank our colleagues, Steven McCue, Neil 

 van den Heever, and Danie van Zyl for technical sup- 

 port. We are also grateful to the skipper and crew of the 

 research vessel Sardinops, which was used to conduct 

 the field trials. J.P.K. received financial assistance from 

 the Fridtjof Nansen and NORAD, and would like to 

 thank his supervisors, Anders Ferno and Geir Blom, at 

 the University of Bergen in Norway, for their assistance 

 with an earlier draft of this manuscript. D.S.S. thanks 

 the University of Port Elizabeth for support in terms 

 of finance and infrastructure. Finally, the constructive 

 comments of three anonymous referees are acknowl- 

 edged; these aided substantially in clarifying certain 

 parts of the original manuscript. 



Literature cited 



Arana, P. E„ and S. V. Ziller. 



1994. Modelling the selectivity of traps in the capture 

 of spiny lobster {Jasus frontalis), in the Juan Fernan- 

 dez archipelago (Chile). Investigation pesq., Santiago 

 38:1-21. 

 Brown, R. S., and N. Caputi. 



1983. Factors affecting the recapture of undersize west- 

 ern rock lobster Panulirus cygnus George returned by 

 fishermen to the sea. Fish. Res. 2:103-128. 

 1985. Factors affecting the growth of undersize west- 

 ern rock lobster, Panulirus cygnus George, returned by 

 fishermen to the sea. Fish. Bull. 83:567-574. 

 Cockcroft, A. C, and A. I. L. Payne. 



1999. A cautious fisheries management policy in South 

 Africa: the fisheries for rock lobster. Mar. Policy 

 23(6):587-600. 

 Crous, H. B. 



1976. A comparison of the efficiency of escape gaps and 

 deck grid sorters for the selection of legal-sized rock 

 lobsters Jasus lalandii. Fish. Bull. S. Afr. 8:5-12. 

 Davis, G. E. 



1981. Effects of injuries on spiny lobster, Panulirus argus, 

 and implications for fishery management. Fish. Bull. 

 78:979-984. 

 Everson, A. R., R. A. Skillman, and J. J. Polovina. 



1992. Evaluation of rectangular and circular escape 

 vents in the northwestern Hawaiian Islands lobster 

 fishery. N. Am. J. Fish. Manag. 12:161-171. 

 Fogarty, M. J., and V. D. Borden. 



1980. Effects of trap-venting on gear selectivity in the 

 inshore Rhode Island American lobster, Homarus ameri- 

 canus, fishery. Fish. Bull. 77:925-933. 

 Heydorn, A. E. F. 



1969. The rock lobster of the South African west coast 

 Jasus lalandii (H. Milne-Edwards): 2. Population stud- 

 ies, behaviour, reproduction, moulting, growth and 

 migration. Invest. Rep. Div. Sea Fish. S. Afr. 71, 52 p. 

 Hunt, J. H., W. G. Lyons, and F. S. Kennedy. 



1986. Effects of exposure and confinement on spiny 



